IMPACTS OF WARMING ON CARBON CYCLING IN NORTHERN PEATLANDS

The potential impacts of climate warming on peatland carbon cycling are subject to ongoing debate. Since the Last Glacial Maximum, northern peatlands have behaved primarily as a net sink for atmospheric carbon, storing up to ~455 Pg C or one-third of the global soil carbon pool. However, the likely fate of this carbon under a warming climate remains a major unanswered question in Arctic science. Previous studies of northern peatlands and soils have focused primarily on the potential for carbon release to the atmosphere through enhanced CO₂ and/or CH₄ gas emissions. The release of dissolved organic carbon (DOC) to streams and rivers has recently emerged as an additional and crucial negative term in the carbon balance of peatlands, one that can cause a net carbon loss in peatlands that would otherwise appear to be a net carbon sink. Furthermore, it appears that high-latitude soils and peatlands are particularly susceptible to temperature-driven increases in DOC production. Here we present extensive new data from previously unstudied Siberian streams and rivers that suggest mobilization of currently frozen, high-latitude soil carbon is likely over the next century in response to predicted arctic warming. We present DOC measurements from 96 watersheds in West Siberia, a region that contains the world's largest stores of peat carbon, exports massive volumes of freshwater and DOC to the Arctic Ocean, and is warming faster than the Arctic as a whole. The sample sites span ~10⁶ km² over a large climatic gradient (~55–68°N), providing data on a much broader spatial scale than previous studies and for the first time explicitly examining stream DOC in permafrost peatland environments. Our results show that cold, permafrost-influenced watersheds release little DOC to streams, regardless of the extent of peatland cover. However, we find considerably higher concentrations in warm, permafrost-free watersheds, rising sharply as a function of peatland cover. The two regimes are demarcated by the position of the -2°C mean annual air temperature (MAAT) isotherm, which is also approximately coincident with the permafrost limit. Climate model simulations for the next century predict near-doubling of West Siberian land surface areas with a MAAT warmer than -2°C, suggesting up to ~700% increases in stream DOC concentrations and ~2.7–4.3 Tg yr^-1 (~29–46%) increases in DOC flux to the Arctic Ocean.